Various types of technology involve the precisely timed injection of a vaporized fluid into an air stream. This technology may be used in such different fields as fuel injection systems and metered dose inhalers. While used for vastly differing applications, these technologies typically would benefit from a reliable mechanism for detecting the flow direction of the air stream into which the vaporized fluid is injected.
For example, metered dose inhalers provide a much-needed drug-delivery method that allows patients to aspirate a “puff” of medication rather than swallow a pill, or drink or inject liquid medication. In some cases, as with medications that directly target the patient's lungs, aspiration enables the medicine to reach the target area more quickly. In addition, aspiration is typically considered to be less painful than other drug-delivery methods.
However, metered dose inhalers typically rely on the user inhaling at the same time as the aforementioned “puff” is expelled. If the user miss-times the moment of inhalation, for example, exhaling at the time the puff is expelled, the user may receive an incorrect dose, or no dose at all. Because the user might not know whether or not a correct dose has been administered, the patient may sometimes be forced to choose between skipping a dose and administering a second dose that could lead to possible over-dosing. Depending upon the particular medication being administered, either of these scenarios could have negative consequences. Thus, implementation of an airflow direction detection mechanism may also be useful in metered dose inhalers.
One previously-described method of airflow speed detection is hot wire anemometry. In general, hot wire anemometry relies on the cooling effect of airflow across a heated wire. Typically, hot wire anemometry uses two wires, a hot wire, which acts as the power dissipative element, and a resistance wire, which acts as a reference element, in a bridge circuit. The output voltages of the two wires in the circuit are held equal by regulating the heating current. When incoming air passes over the hot wire, the control circuit must apply more voltage to keep the hot wire at the original temperature differential. The control unit detects this increase in voltage. The greater the mass flow rate, the greater the voltage required to maintain the temperature differential. Mass flow rate thus may be determined based on the voltage required to maintain the desired temperature differential. However, while detection of mass flow rate is often useful, this method has not been capable of detecting flow direction.